The present invention relates to the field of optical recording systems and their use. More specifically, in one embodiment the invention provides a variety of improved optical recording heads for an optical disc player and/or recorder using holographic optical elements.
Read only, erasable, and write-once/read-many (WORM) optical recording media and systems are well known to those of skill in the art. Such media are usually provided in the form of discs ("CD's") and may be used for storage of large amounts of data in a wide variety of applications such as data storage (in computer systems), audio storage, or video storage. The discs with which most consumers are currently familiar are optical read-only discs used in audio applications. These discs have a reflective surface containing pits which represent audio data in binary form. A detailed description of these pits and how they function is provided by Watkinson, "The Art of Digital Audio," Focal Press, Chapters 13 et al., which is incorporated by reference herein for all purposes.
The standard compact disc player generally operates by focusing a laser beam on the reflective metal through the substrate and then detecting reflected light. The pits increase the optical path of the laser beam by an amount equivalent to a half wavelength, thereby producing destructive interference when combined with other (non-shifted) beams. The presence of data thus takes the form of a drop in intensity of the reflected light. The detection system on a standard player is thus designed to require greater than 70% reflection when no destructive interference occurs and an amplitude less than 30% when data are present. These intensity limits, combined with the focusing parameters, set the criteria for the compact discs and other optical data storage media which can be read or played on such players.
Media on which data can be recorded directly and read directly from have a different configuration and operate under a somewhat different principle. One example is described in U.S. Pat. No. 4,719,615 (Feyrer et al.), which is incorporated herein by reference for all purposes. The medium disclosed in Feyrer et al. includes a lower expansion layer of a rubbery material which expands when heated. The expansion layer is coupled to an upper retention layer which is glassy at ambient temperature and becomes rubbery when heated. Both layers are supported on a rigid substrate. The expansion and retention layers contain dyes for absorption of light at different wavelengths. Data are recorded by heating the expansion layer by absorption of light from a laser beam at a "record" wavelength to cause the expansion layer to expand away from the substrate and form a protrusion or "bump" extending into the retention layer. While this is occurring, the retention layer rises in temperature above its glass transition temperature so that it can deform to accommodate the bump. The beam is then turned off and the retention layer cools quickly to its glassy state before the bump levels out, thereby fixing the bump.
Reading or playback of the data is then achieved by a low intensity "read" beam which is focused on the partially reflecting interface between the retention layer and air. When the read beam encounters the bump, some of the reflected light is scattered, while other portions of the reflected light destructively interfere with reflected light from non-bump areas. The resulting drop in intensity is registered by the detector. Removal of the bump to erase the data is achieved by a second laser beam at an "erase" wavelength which is absorbed by the retention layer and not the expansion layer. This beam heats the retention layer alone to a rubbery state where its viscoelastic forces and those of the expansion layer return it to its original flat configuration. The write, read and erase beams all enter the medium on the retention layer side, passing through the retention layer before reaching the expansion layer.
A variety of improved optical recording systems and media have been devised. Certain of these improved recording systems and media are disclosed in, for example, U.S. Pat. No. 4,879,709 and U.S. application Ser. No. 294,723, both of which are assigned to the assignee of the present invention and incorporated herein by reference for all purposes.
A large number of optical head designs have been introduced in the last few years to serve the needs of the compact disc, optical data storage and video disc market. Each market has its own characteristic requirements. The emphasis in the CD market, for example, is low piece price. Low piece price is often realized by minimizing the number of components at the expense of the lightpath efficiency. Although cost is still an important consideration for optical data storage applications, a high efficiency light path is even more important. Optical heads for video disc applications have been somewhat constrained in their development because of the lack of short wavelength laserdiodes. However, with the growing availability of short wavelength laserdiodes one can expect a revived interest in this area of optical head design.
Conventional optical components made out of glass tend to be expensive because they are made in batches, and human intervention during the manufacturing process can hardly be avoided. One way to solve this problem is application of injection molded lenses. Although molded lenses can be manufactured at very low cost, their mechanical and optical properties are in general of less quality than their comparable glass counterparts. They have been used in some optical heads for CD's but have been avoided in applications where recording of media is required. Another alternative is the application of Holographic Optical Elements (HOE). Previous HOE applications generally have low efficiency and are, therefore, used primarily for CD applications. Previous HOEs tended to act only as a complex grating; they diffract the light but do little else. Many of the holograms used in prior HOEs were computer generated holograms and are implemented as surface holograms. This type of hologram tends to be inefficient and not desirable for optical read/write heads. Furthermore, a single hologram generally performs a single function in the case of surface holograms.
From the above it is seen that an improved optical media recording/reading system and method of recording data on and reading data from optical data storage media are desired.